At the bottom of the mirror, on the back, probably the right side, you'll find a stop that the mirror drops down against. Over time this stop gets hammered down by the returning mirror. When that happens, the mirror travels slightly farther down than it's supposed to, lengthening the total distance from lens to ground glass. You attempt to correct by focusing back slightly farther, and most zoom lenses and all very recent Nikon AF lenses will focus slightly behind infinity to allow for oddities in the AF mechanism of some Nikon cameras. Earlier unaltered lenses and MF lenses stop dead at real infinity, so they can't be focused beyond infinity to compensate for this.
If the mirror is the problem, looking closely at your photos you'll see that they're all very slightly back-focused. This will be most visible at longer distances and wider openings, where a misfocus can mean focus 50 feet behind where intended. On closeups the difference may only be a couple of mm and you might assume it's personal focus error. Almost every older SLR I've ever bought has developed this problem to some extent or another, and that's why there's an easy adjustment for it.
This is all with manual focus, but auto will be affected as well in many cases, depending on the camera. I don't know the F5 so I can't say.
If you lift the mirror up a bit with your finger you'll see the stop, and also that there's an adjustment mechanism to correct the error. The tiniest turn of the adjusting screw is more than enough to correct the error--it's very touchy! The basic strategy is to adjust the mirror so that a lens with known true infinity setting focuses on the ground glass spot on at infinity, using a very distant (a couple of miles) target.
I had an experience with an authorized Nikon service cleaning one of my MF lenses, and they adjusted it to focus beyond infinity because, they said, some cameras won't give the green dot until you have gone past focus and come back, and people were not believing that their cameras were focusing at infinity unless they saw the dot. They said this was according to a Nikon service directive, not their idea. When I complained that I couldn't scale focus anymore, they set it back, so be warned that you need to use a lens in its original state, with confirmed infinity at infinity. Some people have argued with me about this coming from Nikon, but I'm just telling you what this very-respected Nikon service center told me, so don't take it up with me.
